The present invention relates to an antenna arrangement for use in a wireless terminal, for example a mobile phone handset, and to a radio communications apparatus incorporating such an arrangement.
Wireless terminals, such as mobile phone handsets, typically incorporate either an external antenna, such as a normal mode helix or meander line antenna, or an internal antenna, such as a Planar Inverted-F Antenna (PIFA) or similar.
Such antennas are large in relation to a mobile phone handset, but small in relation to a wavelength and therefore, owing to the fundamental limits of small antennas, narrowband and relatively lossy. However, cellular radio communication systems typically have a fractional bandwidth of 10% or more. To achieve such a bandwidth from a PIFA for example requires a considerable volume, there being a direct relationship between the bandwidth of a patch antenna and its volume, but such a volume is not readily available with the current trends towards small handsets. Hence, because of the limits referred to above, it is not considered feasible to achieve efficient wideband radiation from small antennas in present-day wireless terminals.
A further problem with known antenna arrangements for wireless terminals is that they are generally unbalanced, and therefore couple strongly to the terminal case. As a result a significant amount of radiation emanates from the terminal itself rather than the antenna.
An object of the present invention is to provide an improved antenna arrangement for a wireless terminal.
According to a first aspect of the present invention there is provided an antenna arrangement comprising an antenna element adapted for driving against a ground conductor, wherein the antenna element is small relative to a wavelength at operational frequencies of the antenna arrangement and wherein the dimensions of the antenna element are arranged so that, when driven via a matching circuit, the bandwidth of the antenna arrangement is dominated by the antenna element and the ground conductor.
The bandwidth is dominated by the antenna and ground conductor rather than the matching circuit when the impedance of the combination of the antenna element and ground conductor is reasonably well matched to a transceiver. If the mismatch is too great, the bandwidth is dominated by the matching circuit, and in addition losses in the matching circuit become too great for efficient operation.
In an antenna arrangement made in accordance with the present invention, the majority of the radiated power comes from the ground conductor (typically a mobile phone handset case or a printed circuit board ground conductor). Suitable choices of geometry for the antenna element enable the required impedance to be provided while the antenna element remains electrically very small.
Such an antenna arrangement is particularly suitable for dual band operation, being driven via a simple via a dual band matching circuit. One example embodiment is suitable for use at the frequencies employed in GSM and DCS1800 systems.
In one embodiment of the present invention the antenna element comprises a triangular conductor that is significantly wider than its height. Such an element is particularly suitable for use with a mobile phone handset where the width of the antenna element is not particularly important while the height generally needs to be minimised to enable the design of a compact handset. In one example of this embodiment the combined height of the antenna and its associated feed pin is only 11 mm while providing an efficiency of 70% at 1800 MHz (at which frequency 11 mm is approximately 0.07 wavelengths).
According to a second aspect of the present invention there is provided a radio communications apparatus including an antenna arrangement made in accordance with the first aspect of the present invention.
The present invention is based upon the recognition, not present in the prior art, that an antenna and a wireless handset can be considered to be two halves of an asymmetrically fed antenna, and on the further recognition that choice of a suitable geometry for the antenna enables a reasonable impedance match to be achieved.